Compared to conventional heat sealing, utilization of ultrasonic sealing in flexible packaging, offers performance advantages such as the ability: (i) to seal through contamination, (ii) to form narrower seal zones, and (iii) to form seals at higher line speeds and at lower temperature environment. Packaging systems suitable for ultrasonic welding include vertical- or horizontal-form-fill-sealing. Despite these benefits, ultrasonic sealing has a significant drawback. Due to the contact geometry between the horn and the anvil and oscillatory deformation, ultrasonic sealing often leads to significant material outflow and squeeze-out of polymer away from the seal area. This “squeeze-out” phenomenon of ultrasonic sealing can cause damage, and eventual destruction of the laminate structure in the formed ultrasonic seal. Layer steps, i.e., step changes in the thickness of the material between the horn and anvil (such as in gussets and folds, and cross-section fin seals) are particularly susceptible to damage when ultrasonically sealed. Faulty sealing in these areas reduces seal strength (as measured by peeling tests), may destroy the laminate in the seal zone, and create channel leakers resulting in loss of barrier properties in packaging laminates with thin layers of aluminum or barrier polymers, such as ethylene vinylalcohol copolymers (EvOH).
Approaches proposed to mitigate the aforementioned problems include (i) optimization of the contact geometry, especially the energy director and (ii) control of horn displacement during sealing both of which are subject to limitations.
Careful selection of polymers and optimization of the multilayer structures offers an alternate route to mitigate the problems above. There is hence a need to identify polymers and structures that can be sealed ultrasonically and offer a seal strength comparable to that achievable by conventional heat sealing. A need further exists for an ultrasonic sealing process that increases ultrasonic seal strength. A need also exists for strengthened ultrasonic seals that overcome the shortcomings of contact geometry optimization or adjustment of seal force displacement. A need further exists for improved ultrasonic sealable films to meet the demand for more versatile uses of flexible containers.